The influence of disturbing gravitational field on missile hit precision is becoming more and more prominent in recent years. In view of this problem, an accurate solution of trajectory error propagation for ballistic missile’s powered phase is proposed based on the state space perturbation method (SSPM), in which the vertical deflection on the launch point and the disturbing gravity in the flight process are both considered. Firstly, the perturbed state differential equation is derived and the perturbing forces are divided into four parts according to their physical meaning, namely the geometric term, force term, gravitational acceleration coupling term, apparent acceleration coupling term. Numerical simulations have been done for different initial conditions to verify the effectiveness and efficiency of the model presented. MonteCarlo simulation results show that the computational time is about 1/50 of the ballistic integral method, and the relative error of this model is less than 10% compared with the latter.